The present invention relates to a light guide plate (light guiding plate) made of a transparent resin for a liquid crystal display in a personal computer, a cellular phone, PDA (personal digital assistant) and the like or for use in other fields, a molding method of a light guide plate, an insert block suitable for molding the above light guide plate, a mold assembly having the insert block suitable for molding the above light guide plate, and an area light apparatus (surface-emitting light source apparatus) having such a light guide plate.
A liquid crystal display for use in a personal computer, a cellular phone, PDA and the like has an area light apparatus incorporated to cope with demands of a decrease in thickness, a decrease in weight, power saving, higher brightness and higher definition of the liquid crystal display. The area light apparatus generally has a wedge-shaped light guide plate having tapered slanting surfaces. The light guide plate has a flat first main surface and a flat second main surface opposite to the first main surface, and is generally made from a transparent material.
A light source is incorporated into light tools or lights of transportation means such as an automobile, a train, a vessel and an aircraft (for example, a headlight, a taillight, a high-mount stop light, a small light, a turn signal lamp, a fog light, a room lamp, a light for meter panel, light sources housed in various buttons, a destination display lamp, an emergency light, an emergency exit guiding lamp, etc.); various light tools and lights of buildings (for example, an outdoor lamp, an interior lamp, an illuminator, an emergency lamp, an emergency exit guiding lamp, etc.); a street lamp; a signal; a display board; illuminators for a machine and an apparatus; and a lighting portion of a tunnel and an underpass. A reflector is also sometimes provided thereto. These will be sometimes generically referred to as “lighting tool” hereinafter.
In a back-light-type area light apparatus in a liquid crystal display, the area light apparatus is arranged such that a second main surface 344 of a wedge-shaped light guide plate 340 faces a liquid crystal display 60 as its conceptual view is shown in FIG. 21A (in FIGS. 21A and 21B, a concave-convex portion 342 is formed in a first main surface 341. Light that is emitted from a light source 350 and enters a larger-thickness end portion 345 of the wedge-shaped light guide plate 340 is divided into light that is reflected by the first main surface 341 and transmitted from the second main surface 344 and light that is transmitted through the first main surface 341. The light that is transmitted through the first main surface 341 is reflected from a reflection member 351 positioned so as to face the first main surface 341, re-enters the light guide plate 340 and is transmitted from the second main surface 344. Light that is transmitted from the second main surface 344 is introduced to the liquid crystal display 60 positioned so as to face the second main surface 344. Between the liquid crystal display 60 and the second main surface 344 of the light guide plate 340, generally, a stack of a plurality of prism sheets 355 and a plurality of diffusing sheets 352 are arranged and work to uniformly diffuse the light.
In a front-light-type area light apparatus in a liquid crystal display, the area light apparatus is arranged such that a second main surface 344 of a light guide plate 340 faces a liquid crystal display 60, as its conceptual view is shown in FIG. 21B. Light that is emitted from a light source 350 and enters a large-thickness end portion 345 of the wedge-shaped light guide plate 340 is reflected by a first main surface 341 and is transmitted through the second main surface 344. And, the light is allowed to pass through the liquid crystal display 60 arranged in a position facing the second main surface 344, is reflected by a reflection member 354 and is allowed to re-pass through the liquid crystal display 60. This light further passes through a phase-shift film 353 and an anti-reflection layer (not shown) formed on the second main surface 344 of the light guide plate 340 and is transmitted through the first main surface 341 of the light guide plate 340, to be recognized as a screen image. The front-light-type area light apparatus gives a brighter screen than the back-light-type area light apparatus and can attain a bright screen with external light alone if it is a daytime, so that it has an advantage that a power consumption can be decreased.
Meanwhile, a plurality of the prism sheets 355 have problems that they are expensive and that the number of steps for assembling them is large. The above problems are overcome by forming a concavo-convex portion 342 in the first main surface 341 of the light guide plate 340 (see, for example, JP-A-55712/1998). For attaining low power consumption and a higher brightness, it is required to improve the brightness efficiency by increasing the density of the concavo-convex portion 342 having a prism form to the utmost. Further, it is attempted to remove the diffusion sheet by providing the second main surface 344 with an emboss having a light-diffusing effect by blast finishing.
Conventionally, the above light guide plate is formed from a material such as PMMA. However, the heat to be generated inside a machine such as a personal computer, a cellular phone, PDA and the like tends to increase, and the above material is being replaced with a polycarbonate resin having high heat resistance.
However, when the light guide plate 340 having the prism-shaped concavo-convex portion 342 in the surface is formed from a polycarbonate resin having poor flowability by an injection molding method, particularly, there is caused a problem that no prism-shaped concavo-convex portion can be formed in a surface portion of a light guide plate positioned far from a gate portion. It is assumed that the above phenomenon is caused as follows. Since the cavity surface of a mold is made of a metal, a molten polycarbonate resin injected into a cavity is rapidly cooled, and as a result, a solidified layer is formed, and the solidified layer inhibits the formation of the prism-shaped concavo-convex portion 342 in that portion.
For example, JP-A-318534/1996 discloses a method in which an insert block made of zirconia (ZrO2) ceramics including partially stabilized zirconia (partially stabilized zirconium oxide, ZrO2) is incorporated into a mold for molding a molded article made of a thermoplastic resin, to improve the surface appearance, etc., of the molded article. In the mold having a first mold member and a second mold member, disclosed in the above Japanese Laid-open Patent Publication, the insert block is arranged in the first mold member, and a cover plate for protecting the end portion of the insert block is attached to the first mold member so that no load is exerted on the insert block when the first mold member and the second mold member are clamped, to prevent the damage of the insert block. And, the surface of a portion of the insert block facing the cover plate (to be sometimes referred to as “facing surface of the insert block” hereinafter) and the cover plate are arranged to have a clearance of 0.03 mm or less.
When such an insert block is used to form a light guide plate whose surface has a prism-shaped concavo-convex portion, it is required to mechanically machine the surface of the insert block to form a concavo-convex portion in the surface of the insert block. However, it is difficult to form a prism-shaped concavo-convex portion in zirconia ceramics, since the zirconia ceramics has high hardness and is fragile. When an insert block is manufactured from zirconia ceramics and then the concavo-convex portion is mechanically formed in the surface of the insert block, it is difficult to form an intended shape or pattern. Further, when the concavo-convex portion is mechanically formed in the surface concurrently with the formation of the insert block from zirconia ceramics, the insert block is liable to break due to a crack that occurs in the surface.
It is therefore difficult to employ a method in which the concavo-convex portion is formed in the surface of the insert block disclosed in JP-A-318534/1996 and the concavo-convex portion is transferred to the surface of the light guide plate while improving transferability and overcoming non-uniformity in the transfer.
In the technique disclosed in JP-A-318534/1996, the insert block is made of a fragile inorganic material such as ceramics or glass. Particularly, the end portion of the insert block has a form weak in view of strength, so that there is caused a problem that when a stress is exerted on the end portion during assembling of a mold, the insert block is broken. In a mold disclosed, for example, in JP-A-42650/1999, when a surface of an insert block and an insert-block-covering portion, facing such a surface of the insert block, have no sufficient high parallelism during clamping of the mold, there is a risk that the end portion of the insert block comes in contact with the insert-block-covering portion and, as a result, may be broken.
Zirconia ceramics used for the above insert block has hardness at least twice as high as hardness of metal. It is therefore difficult to cut zirconia ceramics with a general metal working machine. Further, when it is cut (machined), the end portion of a cutting tool is liable to escape so that cutting is very difficult. For fitting an insert block in a mold member accurately at a clearance of 0.03 mm or less, therefore, not only is it required to fabricate the insert block with accuracy of ±0.01 mm so that the insert block itself is improved in machining accuracy, but it is also required to delicately machine and adjust a mold portion to which the insert block is to be fitted (insert-block-fitting portion) and an insert-block-covering portion to the dimensions of the insert block. Further, when the insert block is constituted in a three-dimensional form, the problem is that metal machining of a mold portion near the insert-block-fitting portion is very difficult, so that it is required to rely on an ultra-precision working machine or a highly skilled worker. Further, as the machining accuracy of the insert block increases, the production cost of the insert block decreases, and further, the cost of the mold itself increases.
Moreover, when an insert block made of zirconia ceramics is fitted to a mold portion provided with the insert-block-covering portion, or when the insert block is fitted to a first mold member, the insert block may break if the fitting is not highly accurate. A working machine for ceramics and a working machine for metal differ, and such a difference makes the fitting of the insert block difficult.
That is, it is difficult to cut an insert block made of ceramics with a surface cutting machine for metal. It is therefore required to cut the insert-block-fitting portion to the height (or thickness) of the insert block fabricated, or cut the reverse surface of a metal plate bonded to the reverse surface of an insert block with an adhesive, in order to adjust a clearance between the height (or thickness) of the insert block and the insert-block-fitting portion. However, the above machining is complicated. Further, when the thickness of the adhesive is not uniform, the procedure of cutting the reverse surface of the metal plate for adjusting the height (or thickness) of the insert block is required to be carried out several times, so that the number of steps for manufacturing increases and increases a cost. Further, it is required to carry out the cutting with a precision cutting machine for improving the parallelism between the mold parting surface and the insert block. When the parallelism between the mold parting surface and the insert block is poor, there is caused a problem that the insert block may break in a worst case.
If a metal layer having a thickness sufficient for machining with a general metal working machine is formed on the surface of an insert block made of zirconia ceramics, it is no longer necessary to attain extremely high metal machining accuracy of a mold portion near the insert-block-fitting portion. That is, the metal layer formed on the surface of the insert block made of zirconia ceramics is machined with a metal working machine, whereby the insert block can be finely adjusted to a mold or the insert-block-fitting portion.
Since, however, general zirconia ceramics is electrically non-conductive, it is required to employ a chemical vapor deposition method (CVD method) or a physical vapor deposition method (PVD method) such as a vacuum deposition method, a sputtering method, an ion plating method, an ion beam deposition method and an IVD method (ion vapor deposition method) for forming a metal layer on the surface of an insert block, as described in JP-A-34068/1999. By any one of these methods, generally, a metal layer having a thickness of 20 μm or less can be formed on the surface of the insert block. In these methods, however, it is difficult to form any metal layer having a thickness sufficient for cutting with a general cutting machine on the surface of the insert block. Further, the cost of forming the metal layer is high, and another problem is that the adhesion of the metal layer to the surface of the insert block is not so high.
It is thinkable to employ a method of forming a thick metal layer on the surface of an insert block made of zirconia ceramics by an electric plating method. Since, however, general zirconia ceramics is electrically non-conductive as already described, it is impossible to form the metal layer by an electric plating method alone. Further, when the metal layer is formed by an electric plating method, it is required to form an intermediate layer or film on the surface of the insert block so that the metal layer has higher adhesion to the zirconia ceramics.
An insert block made of zirconia ceramics has low strength particularly in an edge portion, and a C plane cut or an R rounded surface finishing is generally retained in the edge portion. However, some insert blocks are required to have a sharp edge portion depending upon the forms of molded articles. In some cases, a parting surface on which a mold clamping force is exerted is required to be constituted of an edge portion of an insert block. In these cases, if the breakage of the edge portion of the insert block can be reliably prevented, molded articles can be considerably improved in the degree of freedom of their forms.
For example, a diamond-shaped concavo-convex portion having a relatively large height and a large pitch of peaks and valleys is formed in a surface of a light-transmitting member constituting a lighting tool typified by a light tool of a transportation means. The lighting tool is designed such that light emitted from a light source is reflected by a reflection member and that the light is collectively reflected or diffuse-reflected by a concavo-convex portion for visual observation of the light in a broad range. However, the concavo-convex portion has a relatively large height and has a large pitch of peaks and valleys, and further, the distance from the light source to the concavo-convex portion differs depending upon peaks and valley regions of the concavo-convex portion, so that non-uniformity in brightness is liable to occur, or light from the source or light reflected by the reflection member cannot always be fully utilized. For overcoming the above problems, the pitch of the concavo-convex portions is decreased, and the number of peaks and valleys of the concavo-convex portion is increased. However, it is difficult to form fine peaks and valleys of the concavo-convex portion in the surface of the light-transmitting member by an injection molding method. Further, the reflection member has a relatively large size (volume), and it is greatly demanded to decrease the lighting tool in size depending upon where the lighting tool is placed.
Further, in the lighting tool typified, for example, by a room lamp, a cover of the lighting tool has a coarse diffusion pattern or a kind of lens pattern for effectively utilizing light from a light source (for example, fluorescent lamp). However, the effect of the diffusion pattern or lens pattern is not so high, and the number of such lighting tools is actually increased to secure brightness indoors or in some other place. However, it cannot be said that the above measures are preferred in view of energy saving. In a room or underpass not exposed to sunlight, further, it is often required to keep a lighting tool or the like on. For a safety reason, for example, an emergency lamp is always kept on. In such cases, it is very important to accomplish effective use of light from a light source from the viewpoint of energy saving.